Fix infinite scroll loading multiple pages — lock until queue drained

Keep _loading=True during the entire staggered append process.
Only unlock after the last post is added to the grid, preventing
reached_bottom from triggering new fetches mid-drain.

booru_viewer/gui/app.py

@@ -821,7 +821,6 @@ class BooruApp(QMainWindow):
             self._status.showMessage(f"{len(self._posts)} results (end)")
             return
         self._shown_post_ids.update(p.id for p in posts)
-        QTimer.singleShot(100, self._clear_loading)
 
         if not hasattr(self, '_append_queue'):
             self._append_queue = []
@@ -862,7 +861,8 @@ class BooruApp(QMainWindow):
         if self._append_queue:
             QTimer.singleShot(50, self._drain_append_queue)
         else:
-            # All done — prefetch
+            # All done — unlock loading and prefetch
+            self._loading = False
             if self._db.get_setting_bool("prefetch_adjacent"):
                 self._prefetch_adjacent(idx)